1. Field of the Invention
The present invention relates to the coating of metal articles to improve their corrosion resistance.
2. Prior Art
The referenced patent and application describe methods of protecting metallic surfaces to render them highly resistant to salt spray corrosion and to abrasion. In accordance with these methods, articles having a metal surface composed typically of aluminum, iron, zinc, brass and copper alloys are electroplated with zinc, cadmium, nickel or chromium, and are then chromated, rinsed and coated with a thermosetting polymer. The chromating step is carried out using a water solution containing about 6 ounces to about 16 ounces per gallon of liquid composed of about 90% CrO.sub.3 and about 10% sodium bisulfate.
The process described in the referenced patent and application can not be used in some applications involving hardened, high carbon steels. Since the processing includes an electroplating step, there is a danger that the high carbon steel will be subject to hydrogen embrittlement.
In the manufacture of steel fasteners used in automobiles seat belt harnesses, for instance, the danger of hydrogen embritlement rules out the use of a corrosion protective process which includes electroplating. A well known alternative is to phosphate the hardened steel, and then apply oil or paint over the phosphate.
In automobile seat belt harnesses, oil cannot be used as it will come into contact with the clothes of the vehicle's occupants. Paint is undesirable from aesthetic and wearability view points. For these reasons, no added protective coating has been applied over the phosphate, and corrosion is often so rapid as to discolor and corrode the fasteners even before the car has been delivered to its owner.
Known phosphating processes typically include the five steps of:
1. Cleaning to metal to be coated with an alkaline cleaner to remove soil, oil, grease, etc.;
3. Rinsing in water;
3. Coating in a phosphate coating bath commonly employing zinc and phosphate compounds in solution that combine to give a zinc phosphate coating;
4. Rinsing once again in water; and
5. Sealing the phosphate coating with a weak chromate acid rinse.